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Jour. Korean Earth Science Society, v. 34, no. 1, p. 102 106, February 2013 http://dx.doi.org/10.5467/jkess.2013.34.1.102 ISSN 1225-6692 (printed edition) ISSN 2287-4518 (electronic edition) Note( 단보 ) 북서태평양아열대고기압지수를이용한북동아시아여름철강수의진단 권민호 * 한국해양과학기술원해양순환기후연구부, 426-744, 경기도안산시상록구해안로 787 Diagnosis of Northeast Asian Summer Precipitation using the Western North Pacific Subtropical High Index MinHo Kwon* Ocean Circulation and Climate Research Division, Korea Institute of Ocean Sciences and Technology, Gyeonggi 426-744, Korea Abstract: The intensity of the East Asian summer monsoon has a negative correlation with that of the western North Pacific summer monsoon. Based on the relationship, we suggest the potential predictability of Northeast Asian summer precipitation by using the relationship. The western North Pacific subtropical high (WNPSH) properly represents the intensity of the western North Pacific summer monsoon. It also dominates climate anomalies in the western North Pacific- East Asian region in summertime. The estimates of the Northeast Asian summer rainfall anomalies using WNPSH variability have a greater benefit than those using the western North Pacific monsoon index. Keywords: monsoon, potential predictability, seasonal prediction, the western North Pacific subtropical high 요약 : 동아시아여름몬순의강도와북서태평양여름몬순의강도는음의상관을갖는것으로알려져왔다. 여기서우리는이관계를이용하여북동아시아여름철강수의잠재예측성을조사하였다. 북서태평양아열대고기압은북서태평양여름몬순을적절히나타내며, 북서태평양-동아시아지역여름철기후편차에주된성분이다. 그리고북서태평양아열대고기압변동성을이용한북동아시아여름철강수편차의추정값은북서태평양여름몬순지수를이용하는것보다더낫다. 주요어 : 몬순, 잠재예측성, 계절예측, 북서태평양아열대고기압 서 론 아시아여름몬순 (summer monoon) 은인도여름몬순, 동아시아여름몬순, 그리고북서태평양여름몬순 *Corresponding author: mhkwon@kiost.ac *Tel: +82-31-400-7761 *Fax: +82-31-408-5829 This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 으로나누어질수있다 (Wang and LinHo, 2002). 우리나라는연중강수량의 70% 이상이여름철에집중되는동아시아여름몬순지역이다. 매해반복되는재해에대한방제를위해서우리나라를포함하는북동아시아지역의여름철강수량변동성의이해는과학적 경제적으로중요하다. 동아시아여름몬순은경년변동성 (interannual variability) 뿐만아니라십년변동성 (decadal variability) 또는수십년변동성 (inter-decadal variability) 을가지고있다 (e.g., Wu and Wang, 2002). 동아시아여름몬순의경년변동성은자체의내적역학뿐만아니라티벳고원 (the Tibetan plateau) 의열적강제력, 서태평양의대

북서태평양아열대고기압지수를이용한북동아시아여름철강수의진단 103 류활동, 엘니뇨, 인도몬순등에의해서영향을받는다 (e.g., Huang, 1985; Nitta, 1987; Wang et al., 2000). 특히북서태평양지역의대류활동은동아시아여름몬순에크게영향을준다 (Lee et al., 2005). 통계모형과대기대순환모형 (Atmospheric General Circulation Model) 을이용하여북동아시아여름철강수를예측하려는많은노력이있었다 (e.g., Kang et al., 1992; Yun et al., 2005). 그러나그예측률은적도부근의지역에비해아직현저히떨어진다 (Wang, 2006). 경계조건해수면온도에따른현재수준의대기대순환모형을이용하여북동아시아지역의여름철강수를직접예측하는것은어려우나상대적으로예측률이좋은북서태평양여름몬순 (the western North Pacific summer monsoon) 의원격상관을이용하면, 북동아시아지역여름철강수의예측성을크게향상시킬수있다. Kwon et al.(2005) 는북동아시아지역여름철강수와북서태평양몬순강도의음의상관성이 1990 년대중반이후크게증가하였음을보였다. 본연구에서는북서태평양아열대고기압 (the western North Pacific subtropical high) 과북동아시아지역여름철강수의상관성을보고자한다. 또한, 북서태평양아열대고기압이정확히예측되었다고가정하였을때, 통계적인방법을이용하여북동아시아지역여름철강수의잠재예측성 (potential predictability) 을제시하고자한다. 자료 분석을위하여월평균 NCEP/NCAR (the National Centers for Environmental Prediction/the National Center for Atmospheric Research) 재분석자료가이용되었다 (Kalnay et al., 1996). NCEP/NCAR 재분석자료의수평격자는위도 2.5 o 와경도 2.5 o 이다. 재분석자료의분석에이용된기간은 1979년부터 2011년까지이다. 강수량자료로월평균 CMAP (the CPC Merged Analysis of Precipitation) 이이용되었다 (Xie and Arkin, 1997). CMAP자료의수평격자는위도 2.5 o 와경도 2.5 o 이고분석에사용된기간은 1979년부터 2009년까지이다. Fig. 1. Composite JJA (June, July, and August) - mean geopotential height anomalies (shaded), and horizontal wind anomalies (vectors) at 850 hpa for (a) the most dominant cluster and (b) the second cluster. Units are geopotential meter and m s 1, respectively. The each cluster was obtained by that the hierarchical cluster analysis was performed for JJA-mean zonal wind anomalies during 1979-2009. 북동아시아여름철강수와북서태평양아열대고기압의관계 동아시아지역여름철하층바람장에대한경년변동성의특징을살펴보기위하여 1979 년부터 2009 년까지 850 hpa 여름철동서바람편차에대하여군집분석 (the cluster analysis) 을수행하였다. 군집분석은자료를소수의군집으로묶는방법으로종관패턴이나예측앙상블의분류에활용될수있다 (Kalkstein et al., 1987). 사용된군집분석방법은계층적군집분석 (the hierarchical clustering analysis) 이며, 패턴과의유사성을나타내는지표로패턴상관계수 (the pattern

104 권민호 Fig. 2. (a) Normalized EASRA (East Asian summer rainfall anomaly), WNPSH (western North Pacific subtropical high), and WNPMI (western North Pacific monsoon index) are represented by bars, closed circles, and open circles, respectively. (b) Sliding correlation coefficients between EASRA and WNPSH in a closed circles (EASRA and WNPMI is an open circles) with a window of 11 years. Horizontal solid line is denoted by 95% confidence level. correlation coefficient) 를이용하였다. 군집분석결과, 전체 31개의패턴중 18개가하나의군집 (CLUST1) 으로묶을수있으며, 10개가또다른군집 (CLUST2) 으로묶을수있었다. 각군집에대한 850 hpa 수평바람및지위고도 (geopotential height) 의합성도가 Fig. 1에도시되었다. 그림에서보이듯이전체 31개의패턴중 28개즉약 85% 가북서태평양아열대고기압과관련된패턴이다. Fig. 1a는북서태평양아열대고기압의음의위상즉, 북서태평양여름몬순의양의위상을나타내고, Fig. 1b는 Fig. 1a의반대위상을나타낸다. 북서태평양여름몬순과북동아시아지역여름철강수를지수화하기위하여 WNPMI (western North Pacific monsoon index; Wang et al., 2001) 와 EASRA (East Asia summer rainfall anomaly; Lee et al., 2005) 가각각이용되었다. 한편, 여름철 850 hpa 지위고도편차의분산은북서태평양일부지역 (115 o E- 150 o E, 15 o N-25 o N) 에서크게나타나는데, 그지역을평균한지위고도편차가북서태평양아열대고기압지수로이용되었다. 편의를위해서이지수를이후 WNPSH (western North Pacific subtropical high) 라고한다. 이세지수가 Fig. 2a에비교되었다. 선행연 Fig. 3. Maps of the correlation coefficients between JJAmean zonal wind anomalies and the most dominant principal components of EOF (Empirical Orthogonal Function) of JJA-mean geopotential height anomaly during (a) 1979-1993 and (b) 1994-2011. Boxed areas indicate regions for definition of WNPMI. 구에서지적된것과같이 WNPMI 는 EASRA 와통계적으로유의한음의상관 ( 상관계수는 -0.47) 을가지고있고 (Lee et al., 2005), WNPSH 와 EASRA 의상관계수는상대적으로더높은 0.60 이다. 한편, WNPMI 와 EASRA 의상관관계는 1990 년대중반을전후로강화되는데 (Kwon et al., 2005), WNPSH 와 EASRA 의상관관계는 1990 년대초반을제외한분석자료의기간에대하여보다더통계적으로유의하다 (Fig. 2b). 두지수가차이가나는이유는북서태평양아열대고기압의수평구조가달라졌기때문이다. Fig. 3 은여름철평균 850 hpa 지위고도편차의 EOF (empirical orthogonal function) 첫번째모드의 PC (principal components) 와여름철평균 850 hpa 동서바람편차의상관도를보인다. 사각형들은 Wang et al.(2001) 에서정의한동서바람영역을의미하는데, 1994 년에서 2011 년까지기간에서는그사각형의영역과상관계수패턴이잘일치하나 1979 년부터 1993 년까지는사각형영역과상관계수패턴이잘일치하지않는다. 즉 WNPSH 의수평구조가기간에따라달라지기때문에 WNPMI 와 WNPSH 의시계열은약간의차이를가질수있다.

북북북북북북북북북북북북북북북북북북북북북북북북북북북북북북 105 변동성의합은전체변동성의약 68.6% 이다. 는 0.5 를이용하였다. 모형의검증은교차검증법 (cross-validation method) 이이용되었다. WNPSH 를이용한모형의상관계수성능은약 0.60 이지만, WNPMI 를이용한모형의상관계수성능은약 0.46 이다 (Fig. 4b). 즉북동아시아여름철강수를예측하는데, WNPSH 가효율적으로이용될수있으며, 비교적높은잠재예측성을가지고있다. 토 의 Fig. 4. Normalized EASRA (open circules) and estimated EASRA (closed circles) using (a) WNPSH, and (b) WNPMI by the additionally constrained linear regression model. See text for details. WNPSH 를이용한 EASRA 의잠재예측성 WNPSH 를이용하여 EASRA 를추정하는데다음과같은간단한선형회귀모형이이용되었다. ŷ = Xa 여기서 ŷ 는예측변수벡터, X는예측인자행렬, 그리고 a는선형회귀계수벡터이다. 한편, 북동아시아지역여름철강수는시공간적으로노이즈가많기때문에선형회귀모형에다음과같은강제조건이추가되었다. ------- [ ε 2 + γ( ŷ ỹ) 2 ] = 0 a T 여기서 ε은관측벡터 (y) 과예측변수벡터의차이이고, γ는상수, 그리고 ỹ 는소수의 EOF모드의재조합 (reconstruction) 된관측벡터이다. 이와같은강제조건이추가되면선형회귀모형의추정정확도는더높아진다고알려져있다 (Weare, 1987). 위방정식을풀면, 선형회귀계수벡터 a 는다음과같이표현된다. a = 1 --------- ( X T X) 1 X T ( y + γỹ) 1 + γ WNPSH 를예측인자로, EASRA 를예측변수로두어위모형을이용하여 EASRA 를예측하였다 (Fig. 4a). 추가적강제조건을위하여 EASRA 지역의강수량을 EOF 하고 5 개모드만재조합하였다. 5 개모드 현재수준의대기대순환모형을이용한중위도지역강수예측률은물리모수화과정등의이유로충분히높지않기때문에, 북동아시아여름철강수를예측하는데통계적인방법이상보적으로이용될수있다. 특히, 대기대순환모형에서예측된북서태평양아열대고기압의변동성을이용하면, 북동아시아여름철강수를더정확히예측할수있다. 북서태평양아열대고기압의기작 (mechanism) 은인도양 - 북서태평양결합모드와라니냐강제모드 (La Nina-forced mode) 로설명할수있어 (B. Xiang 과개인적교류 ), 그변동성을진단하거나예측하기용이하며, 특히대기대순환모형은물론대기 - 해양결합대순환모형에서도그변동성이잘모의된다 (Lee et al., 2008). 북서태평양아열대고기압지수즉, WNPSH 를이용한 EASRA 의잠재예측성은상관계수성능이약 0.60 이지만, 그잠재예측성은대기 / 해양의장기변동성에따라변화할수있다. 본연구에서는대기대순환모형에서예측된 WNPSH 와 EASRA 의관계는고려되지않았다. 향후예측모형의 WNPSH 와 EASRA 의관계를재조명할필요가있으며, EASRA 를직접예측한것과예측된 WNPSH 를이용한예측의예측률을비교하여야한다. 한편북서태평양아열대고기압변동성뿐만아니라, 북동아시아여름철강수량을변화시킬수있는요인은많다. 특히, 북인도양해수면온도변동성이나 EU 패턴 (Eurasian pattern) 은대기의파동을통해북동아시아의여름철강수량변동성에영향을줄수있다 (Lee et al., 2005; Kripalani and Ashwini, 2001). 북인도양해수면온도와 EU 패턴을알수있다면북동아시아여름철강수량을더욱정확히추정할수있다. 계절시간규모에서우리나라를포함하는북동아시아지역여름철강수량을정확히예측하기위하여동아시아여름몬순변동성의지배역학을이해하기위하여

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